"Sexy Sadie"'s opening -- group unknown (6)
I believe the answer is:
steamy
'sexy' is the definition.
(I've seen this before)
'sadie's opening group unknown' is the wordplay.
'opening' suggests taking the first letters.
'group' becomes 'team'.
'unknown' becomes 'y' (algebra).
The initial letter of 'sadie' is 's'.
's'+'team'+'y'='STEAMY'
(Other definitions for steamy that I've seen before include "risque behaviour?" , "Vaporous: lustful" , "Passionate" , "Salacious" , "Hot and humid - erotic" .)